<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <title>css盒模型</title>
  <style>
    html, * {
      margin: 0;
      padding: 0;
    }
    #sec {
      background-color: red;
    }
    #sec > .child {
      height: 300px;
      margin-top: 10px;
      background-color: blue;
    }
  </style>
</head>
<body>
  <section id="sec">
    <div class="child"></div>
  </section>
</body>
</html>